Joint replacement surgery is quite common and enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints are normally composed of metallic and/or ceramic components that are fixed to existing bone.
Knee arthroplasty is a well known surgical procedure by which a diseased or damaged natural knee joint is replaced with a prosthetic knee joint. Typical knee prostheses include a femoral component, a patella component, a tibial tray or plateau, and a tibial bearing member. The femoral component generally includes a pair of laterally spaced apart condylar portions, the inferior or distal surfaces of which articulate with complementary condylar elements formed in a tibial bearing component. U.S. Pat. No. 5,609,643 provides an example of such a knee joint prosthesis and is hereby incorporated herein by reference.
In a properly functioning artificial knee joint, the condylar portions of the femoral component must slide and roll freely over the articulation surface formed by the condylar elements of the tibial bearing member. Natural friction within a replaced, artificial joint can lead to the development of wear debris in which minute particles of debris (e.g., metal or plastic from the prosthesis) become dislodged and migrate within the joint. The phenomenon of wear debris within artificial joints is a serious problem that can inhibit the proper mechanical functioning of the joint. Moreover, wear debris can lead to osteolysis and bone deterioration. When wear debris develops within an artificial joint, surgical removal of the debris or subsequent replacement of the artificial joint is often necessary.
Joint replacement surgery obviously requires a tremendous degree of precision to ensure that prosthetic components are properly sized, implanted, and aligned. The anatomy of patients who undergo knee arthroplasty is widely variable and can lead to difficulty in matching the standard sized prosthetic components that form a prosthetic joint. Imperfect sizing, implantation and alignment can lead to inadequate performance of the knee joint as well as to the presence of high contact stresses in certain areas of the prosthesis, thus leading to the possible development of wear debris.
During normal usage of a properly implanted prosthetic knee joint, load and stress are placed on the tibial bearing member. The tibial bearing member is typically made of an ultrahigh molecular weight polyethylene (UHMWPE), and friction, continuous cycling and stress can cause some erosion or fracture of the tibial bearing member, thus leading to wear debris. The risk of wear debris can be even greater during malalignment of an artificial knee joint, which can result from normal usage or from imperfect or inaccurate implantation of the prosthesis within a patient. Due to malaligniment, the load on the tibial bearing member is not evenly distributed. Instead, excess load is placed on certain areas of the tibial bearing member. Even with the best available condyle designs, varus/valgus malalignment can lead to condylar lifting. That is, one of the femoral condyles is lifted from the tibial bearing member leaving all of the bearing load on one condyle. This uneven distribution of load can accelerate the development of wear debris. Contact stresses on the tibial bearing member increase the risk that wear debris will develop when a prosthetic knee joint is subjected to malalignient conditions.
In addition, conventional knee prostheses have no features which perform the load distribution functions of the medial and lateral menisci. These menisci absorb energy and distribute loads uniformly to the underlying bone. Without an energy absorbing mechanism in the knee prosthesis, shock loads are transmitted directly to the tibial tray-bone interface, leading to a loosening of the tibial tray component of the knee prosthesis. This loosening can lead to further malalignment and instability in the knee joint.
There is thus a need for knee joint prostheses that have a reduced tendency to develop wear debris due to the maintenance of good contact area and low contact stress between femoral and tibial components, even during the dynamics of daily activity and in various conditions of malalignment. There is further a need for knee joint prostheses that can distribute loads uniformly through the joint to the underlying bone.